Schedule III - Real Estate and Accumulated Depreciation (Activity in Real Estate and Accumulated Depreciation)(Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in real estate
Balance at beginning of year	$ 12,743,332	$ 16,681,602
Property acquisitions	13,323	29,230	$ 3,064,137
Improvements	342,014	426,060	396,555
Retirements/disposals/deconsolidation	(2,892,547)	(4,393,560)	(848,231)
Balance at end of year	10,206,122	12,743,332	16,681,602
Aggregate cost of land, buildings and improvements before depreciation	9,200,000
Changes in accumulated depreciation
Balance at beginning of year	2,264,694	2,060,706
Depreciation for year	347,015	353,502	480,523
Retirements/disposals/deconsolidation	(311,593)	(149,514)	(324,982)
Balance at end of year	$ 2,300,116	$ 2,264,694	$ 2,060,706